In order to eliminate reflection of electric power from a load and efficiently supply electric power to the load when electric power is supplied from a high-frequency power source to a plasma load or other load, an impedance matching device is provided between the high-frequency power source and the load so that an output impedance of the high-frequency power source and an impedance of a circuit viewed from an output terminal of the high-frequency power source toward the load are matched. In the present specification, the impedance of the circuit viewed from the output terminal of the high-frequency power source toward the load is referred to as a load-side impedance.
This type of impedance matching device, which has been widely used in the past, is provided with a mechanically operated variable capacitor provided with an operating shaft for adjusting a capacitance thereof, an inductor constituting part of a matching circuit along with the variable capacitor, a motor for operating the operating shaft of the variable capacitor, and a control unit for controlling the motor so as to cause a position of the operating shaft of the variable capacitor to coincide with a target position, and impedance matching is performed in the impedance matching device by controlling the position of the operating shaft of the variable capacitor to coincide with a necessary position thereof for achieving impedance matching, as disclosed in Patent Document 1, for example.
However, when a mechanically operated variable capacitor is used, an ability to increase a matching speed is limited, and a problem therefore arises that an ability of a matching operation to follow variations in impedance deteriorates when high-frequency electric power is supplied to a load in which the impedance frequently varies, such as in the case of a plasma load, reflected power increases, and electric power can no longer be efficiently supplied to the load.
An electronically controlled impedance matching device has therefore been proposed in which, instead of a mechanically operated variable capacitor, a variable capacitor is used that is capable of electronic control and that has a structure in which a plurality of capacitance elements are connected in parallel, each of the capacitance elements being constituted from a capacitor and a capacitance adjustment switch comprising a semiconductor element connected in series to the capacitor, and impedance matching can be rapidly performed by changing a state (ON state or OFF state) of at least one of the capacitance adjustment switches provided to the variable capacitor each time a variation in the load-side impedance is detected, as disclosed in Patent Document 2.